Integrated circuits are formed on the surfaces of semiconductor substrates such as silicon. Semiconductor devices are isolated from one another through the use of isolation structures formed at the surfaces of the respective semiconductor substrates. The isolation structures include field oxides and shallow trench isolation (STI) regions.
A common design objective is to decrease or minimize the required physical size of an integrated circuit. This down-scaling may be facilitated by using STI regions as isolation structures. FIG. 1 illustrates an exemplary STI region. An opening 112 is formed in a substrate 110, for example, by etching. The opening 112 has an aspect ratio, which is equal to the ratio of depth D1 to width W1. This aspect ratio increases as integrated circuits are scaled down in size. For 40 nm technology and below, the aspect ratio may be greater than 7.0. A liner 114 is formed in the opening 112. Next, an oxide, preferably a silicon oxide, is filled into the opening 112, until the top surface of the oxide is higher than the top surface of the substrate 110.
The oxide is filled into the opening 112 using either high-density plasma chemical vapor deposition (HDPCVD, also known as HDP), or a high aspect-ratio process (HARP). HARP is used to fill trenches having relatively high aspect ratios, whereas HDP is used in situations where oxygen content is to be minimized. For example, the HDP technique is used to fill gaps with aspect ratios no greater than 6.0, whereas the HARP technique is used to fill gaps with aspect ratios that are greater than 6.0.
Another distinction between HARP and HDP relates to the concentration of oxygen in the oxide that is filled into the opening 112. In general, HARP provides a higher concentration of oxygen in oxide formed by HARP as compared to oxide formed by HDP. This higher oxygen concentration is a concern when fabricating high dielectric constant metal gate. (HKMG) transistors because the oxygen may change the threshold voltage of the transistor to an unacceptable extent.
If the oxygen migrates or diffuses into a gate stack of the STI semiconductor device where the device comprises an HKMG transistor, the threshold voltage of the HKMG transistor may change significantly. Moreover, if the STI semiconductor device includes a divot, oxygen diffusion into the gate stack will be further enhanced.